1. Field of the Invention
The present invention relates to an exposure process and apparatus using glass photomasks which are used in the field of optical lithography.
2. Description of Related Art
As conventional exposure processes of a type using a photomask, those shown in FIGS. 4A-4E and FIGS. 5A-5E are known. FIGS. 4A-4E and FIGS. 5A-5E are explanatory views of exposure processes for exposing a negative resist and a positive resist, respectively, using glass photomasks.
According to the exposure process of FIGS. 4A-4E, a single glass photomask 10a is used in a single exposure apparatus. The photomask 10a includes a glass substrate 11 having an exposure pattern composed of a light-blocking area 12 and a light-transmitting area 13. A glass substrate 3 to be processed (target substrate) having a metal film 31 and a negative resist 4a stacked thereon is exposed to exposure light through the light-transmitting area 13 (FIG. 4A).
Then, the resist 4a is developed so that an unexposed area of the negative resist 4a is dissolved and removed (FIG. 4B).
Subsequently, the metal film 31 is etched so that the metal film 31 is removed except for an area under the unremoved resist 4a (FIG. 4C).
The resist 4a is then removed so that the metal film 31 of a desired pattern appears (FIG. 4D and FIG. 4E).
The exposure process of FIGS. 5A-5E uses a single glass photomask 10b in a single exposure apparatus. The glass photomask 10b includes a glass substrate 11 having an exposure pattern composed of a light-transmitting area 12 and a light-blocking area 13. A target glass substrate 3 having a metal film 31 and a positive resist 4b stacked thereon is exposed to exposure light through the light-transmitting area 13 (FIG. 5A).
Then, the resist 4b is developed so that an exposed area of the resist 4b is dissolved and removed (FIG. 5B).
Subsequently, the metal film 31 is etched so that the metal film 31 is removed except for an area under the unremoved resist 4b (FIG. 5C).
The resist 4b is then removed so that the metal film 31 of a desired pattern appears (FIG. 5D and FIG. 5E). Such processes as described above are disclosed in, for example, Japanese Unexamined Patent Publication No. HEI 4(1992)-109223.
Each of the glass photomasks 10a, 10b can be disposed in close contact with the target substrate or at a distance of several tens μm to several hundreds μm from the target substrate, or a pattern of the photomask can be projected onto the target substrate.
The conventional exposure processes using a glass photomask are carried out in the manner described above. Therefore, where there is a light-blocking defect 100 in the light-transmitting portion 13 in FIG. 4A, an unexposed area 110 is formed as shown in FIG. 4B and FIG. 4C. Further, where there is the light-blocking defect 100 in the light-transmitting portion 13 in FIG. 5A, an unexposed area 130 is formed as shown in FIG. 5B and FIG. 5C.
As a result, the metal film 31 having a defect 160 of missing a portion of the metal film as shown in FIG. 4D, FIG. 4E or a defect 150 of having a portion of the metal film remaining as shown in FIG. 5D and FIG. 5E is formed. In order to prevent such defects 150, 160, the photomasks 10a, 10b need to be fabricated so as to eliminate the defect 100, which results in a problem that the fabrication cost of the photomasks increases. In general, the glass photomasks have a structure in which a light-blocking pattern such as a chromium film is formed on a transparent glass substrate. The defects in the photomask 10a, 10b are caused by, for example, an air bubble or a damage in the substrate 11 as indicated by the defect 100, a crack in the light-blocking area 12 of the substrate 11, and adhesion of flying dust or the like during the fabrication process.
Particularly in plasma display panels with screens of increasing size, when the photomasks 10a, 10b are used for forming display electrodes (transparent electrodes, bus electrodes), address electrodes or barrier ribs, the substrate 11 would be defective even with only one air bubble in its large area. This results in problems that a defect-free substrate 11 increases the cost and a defective substrate 11 cannot be effectively used.